Yep so if you look at your prepared spells with L, the spells you *prepare* with the different classes, in each of those categories, will use the spellcasting modifier of that class. So when you're looking at the spells you've slotted in for the Wizard, Cleric, Sorcerer section each of them uses their own spellcasting modifier when it's cast. You can add them to custom if you're not sure which is which, such as if you've added the same spell from 2 different classes, and arrange them into groups when you do in order to make it clear.
For scrolls, it will always use your highest level spellcasting class, if you have spellcasting classes. The character described in your initial post would use Wis for their scrolls since Cleric is their highest level spellcasting class. If you had two classes which were tied for highest level (let's say it was a Wiz 6 / Sorc 6 just as a hypothetical), it would use whichever attribute was higher of the tied classes (so in that example either Int or Cha, depending on which is higher).
So your character would use Wis if casting Chain Lightning from a scroll, and Int if they had learned it as a Wizard and prepared it. A Sorcerer mono class only needs to worry about their Cha, but your character needs to either have a good Wis, Int, and Cha, or at a minimum know which spells will use the stats they have which aren't that great, and use spells for those classes which aren't reliant on DC. Typically not too much trouble since they shouldn't need to worry about Dex or Str, so if you're using the Int headband you can get a good Wis, Con and Cha. Hope that helps!
